The invention relates to a method for producing rastered printing forms in which an image pattern is opto-electronically scanned to produce an image signal, and the printing form production is effected by means of an energy beam recording device, controlled by the image signal, wherein the raster points are recorded, by means of a relative movement between the printing form and the recording device and by an additional lateral transverse deflection of the energy beam, in a raster network having arbitrary raster angles.
The invention is of particular use in connection with engraving machines for producing printing forms in accordance with photogravure, i.e. intaglio, techniques. The operating characteristics of an engraving machine, known per se, will therefore initially be described, whereby the objective of the present invention will become apparent.
The pattern to be reproduced, which may contain half tone images and font material, is tensioned upon a rotating scanning cylinder and is scanned point-by-point and line-by-line, utilizing a light spot of the scanning device which travels parallel to the axis of the scanning cylinder. Depending upon the tonal value of the scanned image points, more or less light is reflected by the pattern into the scanning device and is there opto-electronically transformed into an image signal.
The derastering of the half tone pattern is effected by means of a raster signal, with the image and raster signals controlling a recording device which likewise travels in a direction parallel to the axis of the rotating printing cylinder with such recording device engraving a sequence of more or less deep depressions, successively arranged in a photogravure raster network, into the surface of the printing cylinder during the point-by-point and line-by-line recording. The depth of a depression is dependent upon the tonal value of the assigned image point.
The recording device can be an electro-magnetic engraving machine which has an engraving needle functioning as a cutting tool. In the engraving of the depressions, the raster signal produces a vibrating lifting motion of the engraving needle, whereas the image signal determines the penetration depth thereof into the surface of the printing cylinder.
The recording device also can be an energy beam producer, whose energy beam is focused upon the surface of the printing cylinder by electro-optical means. In such arrangement, the engraving of the depressions is effected by local vaporization of the recording medium, with the raster signal determining the pulse sequence, and the image signal values the respective reaction duration of the energy beam upon the surface of the printing medium, during the engraving of a depression. In the reproduction of a multi-colored pattern, an individual printing cylinder is engraved for each color separation. The printing of the different ink printing forms of a color set employed in multicolored reproduction is then effected in a photogravure rotary printing press.
As it is not possible, in practice, to print the respective printing points of the individual color separation exactly one upon the other, moire and undesirable color effects are formed, both of which having an interferring effect when viewed in the finished print.
The visibility or noticeability of moire and other color effects is decreased in printing techniques by the use of raster networks, in the individual color separations of a color set, which are rotated or twisted relative to one another and printed one upon the other. For such a raster rotation, printing cylinders are required in which the individual raster networks are recorded with the desired individual raster angles relative to the recording direction. Where color separations involve the colors "magenta", "cyan", "yellow" and "black", four rigidly defined raster angles are generally required. However, the printing industry strives for the recording of raster networks having arbitrary raster angles, particularly for example in the case where additional colors are to be printed and other printing carriers are to be utilized, or different raster widths are to be printed one upon the other.
Raster networks having freely selectable raster angles can, for example, be produced by a simultaneous engraving of several engraving lines, utilizing a plurality of engraving devices or elements. The engraving devices thus can be arranged one besides the other in the axial direction of the printing cylinder, or they can be distributed about the periphery of the printing cylinder. The engraving, however, becomes particularly simple when the engraving device employs an energy beam, as such beam can be deflected, without inertia, transversely relative to the recording direction.
Such an engraving arrangement is disclosed in German Offenlegungsschrift No. 2,208,658 in which an electron beam producer is employed which is simultaneously actuated by image information and control information in order to deflect the desired deflection of the electron beam.
Due to the controlled deflection and the continuous rotation of the printing cylinder, the focal spot of the electron beam sweeps the surface of the printing cylinder such that a raster network with arbitrary raster angles and depression spacings is achieved.
While a computer supplies the required control information, the image information is derived by a scanning of the pattern. However, for a reproduction with improved freedom from distortion, it is now required that the tonal value for a depression to be engraved be effected at a scanning point geometrically assigned on the pattern.
Consequently, corresponding with the transverse deflection of the electron beam, scanning points lying transversely to the scanning line must also be selected, without inertia, by the scanning device.
Conventional scanning devices do not satisfy such conditions. Consequently, in such engraving arrangements, the pattern is therefore scanned point-by-point and line-by-line in the known manner with the aid of a conventional scanning device prior to the recording of the pattern, and the image information of the total pattern thus obtained stored in a digital store. In the recording operation, a computer then controls the supply of the image information, image point by image point, from the digital store in the sequence prescribed by the raster arrangement and the raster angle, whereby the corresponding control information is added to the read-out image information.
In this type of engraving arrangement, a reproduction can therefore be made only in two separate steps successively effected. A synchronous scanning and recording is not possible, which is considered an important disadvantage. Further a large store capacity and epensive control devices are necessary.